Fluid actuators

ABSTRACT

A fluid actuator is described in which the displaceable member, e.g., a diaphragm or bellows, is bi-stable so that in a system, e.g., an air pressure system, utilizing a pair of such actuators, the slave actuator can be maintained in either position without requiring a force to maintain on the master actuator. The bistable effect can be achieved by the resilience of the bellows or diaphragm itself or by a separate spring.

United States Patent 1 Tracey [541 FLUID ACTUATORS [76] Inventor: Peter M. Tracey, Crofts Place, Little Saxham, Bury St. Edmunds, England [22] Filed: Nov. 28, 1969 [21] Appl. No.: 880,742

[30] Foreign Application Priority Data Nov. 29, 1968 Great Britain ..56,858/68 [52] US. Cl. ..60/54.5 R, 60/625, 92/103 M, 200/83 B [51] Int. Cl. ..FlZb 7/00, Flb 19/00 [58] Field of Search ..60/62.5, 54.5; 92/43, 48, 98, 92/103; 74/96, 100; 251/75; ZOO/83.2 B,

[ 1 Jan. 16,1973

2,651,491 9/1953 Ashton et a1. ..251/75' 3,174,500 3/1965 Johnson et al.... ..251/75 2,503,854 4/1950 Trainor 192/89 B 1,684,530 9/1928 Bast 92/103 M 2,915,016 12/1959 Weaver et al........ /545 R 2,960,882 11/1960 Simmonds ..74/100 2,993,965 7/1961 Morgan 60/545 R 3,409,224 11/1968 Harp etal ..138/121 3,504,849 4/1970 Quinn ..251/

Primary ExaminerEdgar W. Geoghegan Assistant ExaminerA. M. Zupcic Att0rneyKeny0n & Kenyon Reilly Carr & Chapin 5 7 ABSTRACT A fluid actuator is described in which the displaceable member, e.g., a diaphragm or bellows, is bi-stable so that in a system, e.g., an air pressure system, utilizing a pair of such actuators, the slave actuator can be maintained in either position without requiring a force to maintain on the master actuator. The bi-stable effect can be achieved by the resilience of the bellows or diaphragm itself or by a separate spring.

25 Claims, 6 Drawing Figures PATENTEDJAHHIG 6 ms SHEET 1 OF 3 INVENTOR PETER M TRACE) 49y flTTORNE Y5 rum) ACTUATORS PRIOR ART This invention relates to fluid actuators wherein a chamber for containing working fluid has a member which is displaceable between first and second positions to change the volume of the chamber either in response to change in the pressure of the working fluid or in response to an external force applied to the member in order to change the pressure of the working fluid. It is known that the member may be biassed towards one of said positions so that, except in said one position, the member is unstable. The biassing may be achieved by a spring acting on the member, which may be a piston sliding in a cylinder, the piston and cylinder together defining the chamber.

Such actuators are used in pairs, the chambers being interconnected by a conduit and the system being filled with working fluid. One of the actuators is arranged so that its member may be displaced in response to an external force to compress the working fluid in its chamber, such actuator being designated the master, and the other actuator, designated the slave, is arranged so that its member is displaced in response to the increase in pressure produced in the working fluid by the master actuator. In this way, various operations can be effected from a remote location.

BACKGROUND TO THE INVENTION The problem which arises in this type of system is that since the displaceable member of the actuator is biassed to return to one position, it is necessary to maintain the externally applied force during the period that it is required that the member of the slave actuator should remain displaced. In the case where the working fluid is air, the further problem arises that unless the system is completely air-tight air escapes from the system as a result of which loss of air pressure takes place permitting the displaceable member of the slave actuator to return to its stable position. Thus, remotely controlled operations requiring the displaceable member of the slave actuator to be maintained in its unstable position for a period of time cannot be carried out. Avoidance of the problem by providing a completely air-tight system is difficult and would require precision in manufacture and consequent expense.

THE INVENTION The invention solves these problems by providing a fluid actuator which is characterized in that the displaceable member is biassed into the first position when at one side of an intermediate position and into the second position when at the opposite side of the intermediate position, so that the member is stable in both its first and second positions. Hence, in a system using a pair of the actuators according to the invention, it is unnecessary to maintain an external force on the displaceable member of the master for extended periods of time or to ensure that the system is absolutely air-tight, when air is the working fluid: all that is necessary is for the system to be sufficiently air-tight that the required air pressure can be maintained during the short period in which the displaceable members of the actuators are moving between their stable positions. This is especially advantageous where the system is to be used in an environment in which the ambient temperature varies, since by arranging that the system is not completely air-tight, pressure changes in the system due to temperature changes are avoided, any excess pressure tending to develop being lost through the inherent leakyness of the system.

DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are illustrated in the accompanying drawings, in which:

FIG. 1 shows a system according to one embodiment of the invention in one of its stable positions;

FIG. 2 shows the system in an unstable position;

FIGS. 3 and 4 illustrate second and third embodiments of the invention; and

FIGS. 5 and 6 illustrate master and slave actuators respectively according to a fourth embodiment of the invention.

The system illustrated in FIGS. 1 and 2 comprises two similar fluid actuators 2, 2'. These actuators each comprise a base 4, 4' in the form of a circular plate (e.g. of plastics), at the center of which an axially extending sleeve 6, 6' is formed. One end 8, 8 of the sleeve is open while the other end 10, 10' is closed. Each circular plate is also formed with a nipple 12, 12 extending from the side opposite to that at which the open end 8 of the sleeve 6 is located, and a bore 14, 14' which extends through the circular plate 4, 4' and the nipple l2, 12'.

A displaceable member in the form of a disc 16, 16' of smaller diameter than plate 4, 4', is provided with an axially extending central spindle l8, 18', which is a loose fit in the sleeve 6, whereby the disc 16, 16 can be moved axially towards and away from the plate 4, 4.

The disc 16 has a cylindrical knob 20 fixed to it at the side opposite to the spindle 18. Each disc l6, 16' has a peripheral groove 22, 22.

A tubular wall 24, 24 of flexible material having some resilience, such as PVC, and which is of undulatory configuration in longitudinal crosssection, and has a generally conical shape at one end 25, 25, also interconnects the disc 16, 16 with the plate 4, 4'. The periphery of the plate 4, 4' is engaged with the inside of a circumferential crest 26, 26' of the tubular wall 24, 24, whereas the disc 16, 16 has its peripheral groove 22, 22' engaged with the inside of a trough portion 28, 28, or inner hinge. The dimensions of the plates 4, 4 and discs 16, 16' relative to the crest portions 26, 26' and trough portions 28, 28 are such that the material of the tubular wall 24, 24' is stressed in tension at these portions. Intermediate the ends, the tubular wall 24, 24' has a further crest 30, 30', or outer hinge, and trough 32, 32'. The plate 4, 4, disc 16, 16' and tubular wall 24, 24' define a chamber for working fluid, which in this case is air.

The resilience of the tubular wall 24, 24 biases the disc 16, 16' into either of two end positions and renders it unstable when in an intermediate position. FIG. 1 shows the disc 16 in one of its end positions in which it is remote from the plate 4 and shows the disc 16 in the opposite end position, in which it is close to the plate 4. The movement of the disc 16 from one position to the other, using the knob 20, causes, through the medium of the air in the system, movement of the disc 16' from one position to the other. The nipples 12, 12 being interconnected by a flexible tube 34, whereby the actuator 2 constitutes a master bellows and the actuator 2' a slave bellows. In use, the disc 22' will engage or move the mechanism which is to be operated from a remote location by the system. The mechanism may be, for example, a microswitch or electrical contact.

In moving the disc 16 from its position as shown in FIG. 1 towards its other position, the tubular wall folds at the crest 30 and forces the crest 30 outwards, The stress so produced in the material at the crest 30 initially tends to return the disc 16 towards the position shown in FIG. 1. However, with continued movement of the disc 16, it passes an over-center position and the crest 30 then begins to contract, so that the disc 16 is urged towards the plate 4 and tends to assume the position of the parts of the device 2 as shown in FIG. 1. The intermediate position of the discs l6, 16' in which they are unstable, is shown in FIG. 2, whereas FIG. 1 shows the disc 16 in one of its stable positions and the disc 16' in its opposite stable position, the tubular wall 24 of the device 2' being folded upon itself at the crest 30'. The undulatory configuration of the tubular wall 24,24 ensures that it cannot collapse when a reduced pressure is inside it. This applies particularly to the tubular wall 24' when the disc 16' is being drawn, by vacuum in the tubular wall 24', to the position shown in FIG. 1. In other words the undulatory configuration ensures that the folding of the tubular wall takes place at the desired portion.

The sleeves 6, 6' and spindles 18, 18 simply serve as guides to ensure that the plate 4, 4' and disc 16, 16 remain parallel to one another. It would, however, be possible to dispense with this guide arrangement.

In testing the device illustrated in the drawings, it has been found that the relative dimensions of the tubular wall 24, 24', plates 4, 4' and disc l6, 16' have to be carefully chosen to ensure reliable snap action. These dimensions can be selected by trial and error.

By virtue of the fact that the tubular wall 24, 24' snaps the member l6, 16 into oneor other of its two positions, pressure is only required to be in the system while actually moving the member 16 from one position to the other to cause expanding movement of the member 16'. That is to say, if the member 16' is connected to an electric switch, for example, it is not necessary while holding the switch in either its on or of position to maintain pressure in actuator 2 or 2'. Hence, there is no necessity for making these actuators and the joint between the tube 34 and'the nipples 12,

12' absolutely air-tight, provided of course that the pressure in the system can be maintained long enough for the disc 16' to be snapped from'oneposition to the other. In fact, it is preferable for the system to be such thatif there is any change in ambient temperature or pressure, the pressure inside the system may, by leakage, remain at atmospheric pressure. For this purpose a bleeder aperture may be provided.

Referring to FIG. 3, the system thereshown comprises a pair of actuators 50, 50' which are similar to one'another. Each comprises a housing 52, 52', circular in plan view and dished at 54, 54', and a circular plate spring 56, 56 whose edges are engaged in a circular groove 58, 58'. The circular plate spring 56, 56' is bowed, the housing 52, 52' and spring 56, 56' thus defining a chamber for working fluid, again air. The plate spring 56 carries a knob 59 whereby it can be displaced by manual force between an outward position in which the working chamber is expanded and an inward position in which the chamber is contracted. Actuator 50 is shown with the plate spring 56 in the outward position and actuator 50 with the plate spring 56 in the inward position. The plate spring 56, 56' thus constitutes a wall of the working chamber which is displaceable and biassed to each position by its own resilience and is unstable when between said positions.

The working chamber of each actuator is in communication with a nipple 60, 60. A flexible tube 61 interconnects the nipples 60, 60' whereby inward and outward movement of the plate spring 56 causes outward and inward movement of the plate spring 56' respectively by pressure and suction. The actuator 50 is thus the master and the actuator 50' is thus the slave. In operation the plate spring 56' engages the mechanism to be actuated. The housings and plate springs may be of plastics material or rubber, for example.

In FIG. 4, parts equivalent to parts in FIG. 3 are given the same reference number. The working chamber is formed by a separate bellows 62, 62

mounted in a housing 64, 64' to be compressible by displacement of a plate spring 66, 66 from an outward to an inward position. The bellows 62, 62' has an end wall portion secured at 63, 63', as by adhesive or welding, to the plate spring 66,66 to ensure that the end wall portion is displaced with movement of the spring. The spring is bowed to provide the required snap action or biassing to the inward and outward position and instability in between. The interior of the housing 64, 64 is vented to atmosphere by an aperture 68, 68.

InFIG. 5 the master actuator is located within a plastic casing 70 having the appearance of an ordinary domestic electric light switch and comprises a rigid base 72 and a wall 74 which defines together with the base 72 a working chamber. The wall 74 is circular as seen in thedirection of the arrow A and has a rim 76 engaged in a groove 78 of the base 72. A'lever 80, pivotally mountedin an opening in the casing 70 in the manner of an ordinary domestic electric light switch, is coupled at one end to the center of the wall 74 by a stud 82. The wall 74 is flexible so that it can be displaced, by pivotting of the lever 80, from the inward position shown toan outward position and is resilient so as to be biassedto. each position and unstable in between, thus providing a snapl action. The base 72 is provided with a'nipple.84 for connection by a suitable conduit to aslave actuatorfor actuating alight switch. v g

A suitable form of slave actuator is shown in FIG. 6.

This is similar to the master actuator andiequivalent parts are marked with the. same reference numerals but with a dash added. The slave actuator is carried in" a casing which contains electric terminals 92, for receiving the ends of electrical conductors, and a pair of fixed electrical contacts 94 which are insulated from one another but connected electrically to the respective terminals 92. The circuit between the two contacts 94 can however be completed by a moving contact 96 supported at the center of the wall 74' and so. arranged that when the wall 74' is in its outward position as shown in FIG. 5 the circuit is completed'whereas when in its inward position the circuit is broken.

A pin 98 in the wall 74' acts as a guide for movement of the wall by engagement in the bore leading to the nipple 84'.

Hence, when the master and slave actuators of FIGS. 5 and 6 are interconnected, making or breaking of the switch contacts 94, 96 can be achieved by pivotting the lever 80 with finger pressure.

Although in all of the illustrated embodiments the actuator is provided with a flexible wall which constitutes a bellows or diaphragm, the invention is applicable to other forms of actuators such as a piston and cylinder arrangement or an arrangement in which a plunger is movable inwardly and outwardly of a working chamber.

Also, the invention is applicable to systems using working fluid other than air, such as liquid.

What we claim is:

l. A bistable remote control apparatus for displacing a mechanism between first and second positions and maintaining said mechanism stable in either of said positions, comprising a master fluid actuator adapted to be disposed at a first location; a slave fluid actuator adapted to be disposed at a second location remote from said first location; and conduit means interconnecting said actuators; said conduit means and said actuators containingworking fluid and each said actuator comprising means defining a chamber containing said working fluid, a member associated with said chamber displaceable between first and second positions to change the volume available for said working fluid within said chamber, and resilient means biassing said member into the first position when at one side of an intermediate position and into the second position when at the opposite side of an intermediate position, said resilient means thereby rendering said member stable in both said first and said second positions and substantially unstable when therebetween, said member of said master fluid actuator being displaceable in response to an external force applied thereto and said member of said slave fluid actuator being displaceable in response to changes in the pressure of the working fluid produced by displacement of said member of said master fluid actuator.

2. Apparatus according to claim 1, wherein said working fluid is air, and wherein at least one of said actuators and said conduit means has a bleeder aperture therein whereby said apparatus is incompletely air tight so that ambient temperature variations do not cause the pressure within said conduit and said chambers to depart from atmospheric pressure.

3. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexibility and resilience and wherein said wall constitutes said member, said flexibility permitting said displacement and said resilience effecting said biassing.

4. Apparatus according to claim 3, wherein said chamber defining means comprises a housing and a plate spring mounted on said housing, said plate spring constituting said wall.

5. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexi' bility and resilience, said wall supporting said member, said flexibility permitting said displacement of said member and said resilience effecting said biassing of said member.

6. Apparatus according to claim 5, wherein said wall is tubular, the movement of said displaceable member being axially of said tubular wall and being accompanied by a folding and unfolding of said wall.

7. Apparatus according to claim 6, wherein said wall is of undulatory configuration as seen in longitudinal cross-section having trough and crest portions, said displaceable member being supported by said wall at a trough portion thereof.

8. Apparatus according to claim 3, comprising guide means connected to said displaceable member to control movement of said displaceable member.

9. Apparatus according to claim 8, wherein said chamber defining means includes a first guide member and wherein said guide means is a second guide member secured to said displaceable member, one of said guide members being a pin and the other of said guide members being a sleeve in which the pin is engaged slideably.

10. Apparatus according to claim 9, wherein said sleeve is constituted by an orifice formed in said chamber defining means for the passage of working fluid to and from said chamber.

11. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexibility, said wall constituting said member and said flexibility permitting said displacement, and wherein said biassing means comprises a bowed spring connected to said flexible wall.

12. Apparatus according to claim 1, wherein a knob is carried by said displaceable member of the master fluid actuator for effecting said displacement thereof.

13. Apparatus according to claim 1, wherein a manually operable lever is connected to said displaceable member of said master fluid actuator for effecting the displacement thereof.

14. Apparatus according to claim 1, in combination with an electric switch coupled to the displaceable member of said slave fluid actuator so as to be opened and closed in response to displacement of the displaceable member of the master fluid actuator, said resilient means thereby maintaining said switch stable both in the open position and in the closed position and substantially unstable when therebetween.

15. Apparatus according to claim 14, including a housing containing said master fluid actuator, a lever pivotally mounted in said housing and operable by finger pressure from the exterior of the housing, said lever being coupled to the displaceable member of said master fluid actuator to effect said displacement thereof in response to finger pressure on said lever, and said housing and said lever having the external appearance of a wall mounted domestic light switch.

16. Apparatus according to claim 14, wherein in said slave actuator is mounted in a housing, and wherein said electric switch comprises a pair of fixed contacts carried on said housing and a movable contact carried on said displaceable member and arranged to electrically bridge said fixed contacts when the displaceable member is in one of its positions and to be spaced from said fixed contacts when said displaceable member is in the other of its positions.

17. Apparatus according to claim 1, in combination with a mechanism to be controlled by said apparatus, said mechanism including a part fixed directly to the displaceable member of said slave fluid actuator to be maintained stable in one or other of two positions in response to displacement of the displaceable member of the master fluid actuator. I v

18. Apparatus according to claim comprising guide .means connected to said displaceable member to control the movement of said displaceable member.

"19. Apparatus according to claim 18, wherein said chamber defining means includes a first guide member and wherein said guide means is a second guide member secured to saiddisplaceable member, one of said guide members being a pin and the other of said guide members being a sleeve in which the pin is engaged slideably'.

in a fluid actuator comprising a tubular wall having flexibilityand resilience and defining a chamber for containing working fluid, and a member supported on said walland displaceable axially of saidtubular wall between first and second positions to change the volume available for working fluid within the chamber in responseto change in the pressure of the working intermediate position, whereby themember is stable in both said first and said second positions with said flexibility of said wall permitting the displacement of said member and said resilience effecting the biasing of said member. I

"21; The improvementof claim 20, wherein said wall is of undulatory configuration as seen in longitudinal cross-section having trough and crest portions, said displaceable member being supported by said wall at a trough portion thereof.

' 22. The improvement of claim 21, comprising guide means connected to said displaceable member to con trol movement of said displaceable member.

23. A fluid actuator comprising means defining a chamber for containing working fluid and port means in said chamber defining means for theentry and exit of working fluid, said chamberdefining means comprising a support, a flexible and resilient conical wall member having inner and outer peripheries, an outer hinge extending around said outer periphery of said wall member and hingeably connecting said wall member to said support, an'inner hinge extending around and connected to said inner periphery of said wall member, and a displacement member supported by said inner hinge, said hinges and said flexibility of said wall member permitting movement of said displacement member between a first position where said inner periphery is at one side of an intermediate position and a'second position where said inner periphery is at the opposite side 'of said intermediate position and said resilience of said wall member acting on said displacement member in a direction towards said first position when said inner periphery is at one side of said intermediate position and towards said second position when said inner periphery is at the opposite side of said intermediate position, and including guide means retainingsaid displacement member m a predetermined orientation throughout said movement and when in each of said first and second positions. i

24. Apparatus according to claim 23, whereinsaid displacement member, said conicalwall member and said hinges are all integral with oneanother. r

25. Apparatus according to claim 23, wherein said displacement member is non-integral with said hinges and said conical wall member and is made of substantially rigid material. 

1. A bistable remote control apparatus for displacing a mechanism between first and second positions and maintaining said mechanism stable in either of said positions, comprising a master fluid actuator adapted to be disposed at a first location; a slave fluid actuator adapted to be disposed at a second location remote from said first location; and conduit means interconnecting said actuators; said conduit means and said actuators containing working fluid and each said actuator comprising means defining a chamber containing said working fluid, a member associated with said chamber displaceable between first and second positions to change the volume available for said working fluid within said chamber, and resilient means biassing said member into the first position when at one side of an intermediate position and into the second position when at the opposite side of an intermediate position, said resilient means thereby rendering said member stable in both said first and said second positions and substantially unstable when therebetween, said member of said master fluid actuator being displaceable in response to an external force applied thereto and said member of said slave fluid actuator being displaceable in response to changes in the pressure of the working fluid produced by displacement of said member of said master fluid actuator.
 2. Apparatus according to claim 1, wherein said working fluid is air, and wherein at least one of said actuators and said conduit means has a bleeder aperture therein whereby said apparatus is incompletely air tight so that ambient temperature variations do not cause the pressure within said conduit and said chambers to depart from atmospheric pressure.
 3. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexibility and resilience and wherein said wall constitutes said member, said flexibility permitting said displacement and said resilience effecting said biassing.
 4. Apparatus according to claim 3, wherein said chamber defining means comprises a housing and a plate spring mounted on said housing, said plate spring constituting said wall.
 5. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexibility and resilience, said wall supporting said member, said flexibility permitting said displacement of said member and said resilience effecting said biassing of said member.
 6. Apparatus according to claim 5, wherein said wall is tubular, the movement of said displaceable member being axially of said tubular wall and being accompanied by a folding and unfolding of said wall.
 7. Apparatus according to claim 6, wherein said wall is of undulatory configuration as seen in longitudinal cross-section having trough and crest portions, said displaceable member being supported by said wall at a trough portion thereof.
 8. Apparatus according to claim 3, comprising guide means connected to said displaceable member to control movement of said displaceable member.
 9. Apparatus according to claim 8, wherein said chamber defining means includes a first guide member and wherein said guide means is a second guide member secured to said displaceable member, one of said guide members being a pin and the other of said guide members being a sleeve in which the pin is engaged slideably.
 10. Apparatus according to claim 9, wherein said sleeve is constituted by an orifice formed in said chamber defining means for the passage of working fluid to and from said chamber.
 11. Apparatus according to claim 1, wherein said chamber defining means comprises a wall having flexibility, said wall constituting said member and said flexibility permitting said displacement, and wherein said biassing means comprises a bowed spring connected to said flexible wall.
 12. Apparatus according to claim 1, wherein a knob is carried by said displaceable member of the master fluid actuator for effecting said displacement thereof.
 13. Apparatus according to claim 1, wherein a manually operable lever is connected to said displaceable member of said master fluid actuator for effecting the displacement thereof.
 14. Apparatus according to claim 1, in combination with an electric switch coupled to the displaceable member of said slave fluid actuator so as to be opened and closed in response to displacement of the displaceable member of the master fluid actuator, said resilient means thereby maintaining said switch stable both in the open position and in the closed position and substantially unstable when therebetween.
 15. Apparatus according to claim 14, including a housing containing said master fluid actuator, a lever pivotally mounted in said housing and operable by finger pressure from the exterior of the housing, said lever being coupled to the displaceable member of said master fluid actuator to effect said displacement thereof in response to finger pressure on said lever, and said housing and said lever having the external appearance of a wall mounted domestic light switch.
 16. Apparatus according to claim 14, wherein in said slave actuator is mounted in a housing, and wherein said electric switch comprises a pair of fixed contacts carried on said housing and a movable contact carried on said displaceable member and arranged to electrically bridge said fixed contacts when the displaceable member is in one of its positions and to be spaced from said fixed contacts when said displaceable member is in the other of its positions.
 17. Apparatus according to claim 1, in combination with a mechanism to be controlled by said apparatus, said mechanism including a part fixed directly to the displaceable member of said slave fluid actuator to be maintained stable in one or other of two positions in response to displacement of the displaceable member of the master fluid actuator.
 18. Apparatus according to claim 5 comprising guide means connected to said displaceable member to control the movement of said displaceable member.
 19. Apparatus according to claim 18, wherein said chamber defining means includes a first guide member and wherein said guide means is a second guide member secured to said displaceable member, one of said guide members being a pin and the other of said guide members being a sleeve in which the pin is engaged slideably.
 20. In a fluid actuator comprising a tubular wall having flexibility and resilience and defining a chamber for containing working fluid, and a member supported on said wall and displaceable axially of said tubular wall between first and second positions to change the volume available for working fluid within the chamber in response to change in the pressure of the working fluid or in response to an external force applied to the member in order to change the pressure of the fluid, the movement of said member being accompanied by a folding and unfolding of said wall, said member being unstable when between said positions; the improvement of: means biasing said member into the first position when at one side of an intermediate position and into the second position when at the opposite side of an intermediate position, whereby the member is stable in both said first and said second positions with said flexibility of said wall permitting the displacement of said member and said resilience effecting the biasing of said member.
 21. The improvement of claim 20, wherein said wall is of undulatory configuration as seen in longitudinal cross-section having trough and crest portions, said displaceable member being supported by said wall at a trough portion thereof.
 22. The improvement of claim 21, comprising guide means connected to said displaceable member to control movement of said displaceable member.
 23. A fluid actuator comprising means defining a chamber for containing working fluid and port means in said chamber defining means for the entry and exit of working fluid, said chamber defining means comprising a support, a flexible and resilieNt conical wall member having inner and outer peripheries, an outer hinge extending around said outer periphery of said wall member and hingeably connecting said wall member to said support, an inner hinge extending around and connected to said inner periphery of said wall member, and a displacement member supported by said inner hinge, said hinges and said flexibility of said wall member permitting movement of said displacement member between a first position where said inner periphery is at one side of an intermediate position and a second position where said inner periphery is at the opposite side of said intermediate position and said resilience of said wall member acting on said displacement member in a direction towards said first position when said inner periphery is at one side of said intermediate position and towards said second position when said inner periphery is at the opposite side of said intermediate position, and including guide means retaining said displacement member in a predetermined orientation throughout said movement and when in each of said first and second positions.
 24. Apparatus according to claim 23, wherein said displacement member, said conical wall member and said hinges are all integral with one another.
 25. Apparatus according to claim 23, wherein said displacement member is non-integral with said hinges and said conical wall member and is made of substantially rigid material. 